1. Field of the Invention
The present invention relates to an electronic toll collection (ETC) system for an intelligent transport system (ITS).
2. Description of the Related Art
As shown in FIG. 1 of the accompanying drawings, an ETC system is a road-to-vehicle communication system for collecting expressway tolls from running vehicles via radio communications between a vehicle-mounted device 50 on a vehicle and a road radio unit 100 installed in a toll gate. A conventional radio communication process which is carried out by the vehicle-mounted device 50 on the vehicle that has entered an expressway, for radio communication with the road radio unit 100 in the ETC system will be described below.
As shown in FIG. 1, the road radio unit 100 in the ETC system normally uses two frequencies, i.e., frequencies F1, F2. These two frequencies F1, F2 are used to avoid radio wave interference between plural lanes at entrance and exit toll gates. The frequencies F1, F2 are assigned respectively to adjacent ones of the lanes, and the same frequency is not assigned to successive ones of the lanes. The road radio unit 100 has a narrow communication range in order to avoid radio wave interference between the lanes.
In order for the vehicle-mounted device 50 to effect normal radio communications with the road radio unit 100 at a toll gate or lane of an expressway, the vehicle-mounted device 50 needs to perform bi-directional data communications with the road radio unit 100 within a short period of time of several 100 msecs. in which the vehicle-mounted device 50 runs into and out of the narrow communication range of the road radio unit 100. Therefore, the vehicle-mounted device 50 has to detect, at a high speed, that it enters the communication range of the road radio unit 100. However, since the vehicle-mounted device 50 cannot recognize, in advance, the lane along which the vehicle enters a toll gate, the vehicle-mounted device 50 is unable to know, in advance, which of the frequencies F1, F2 the carrier transmitted from the road radio unit 100 is using. Consequently, the vehicle-mounted device 50 carries out a frequency search process for switching between the frequencies F1, F2 at a high speed so that the vehicle-mounted device 50 can detect the carrier of either one of the frequencies F1, F2 transmitted from the road radio unit 100 whenever the vehicle-mounted device 50 may enter the communication range of the road radio unit 100. The vehicle-mounted device 50 carries out the frequency search process at all times even if it is outside of the communication range of the road radio unit 100.
An arrangement of the conventional vehicle-mounted device 50 will be described below with reference to FIG. 2 of the accompanying drawings. As shown in FIG. 2, the conventional vehicle-mounted device 50 comprises a controller 41, a receiver 43, and an RF (Radio Frequency) module 4.
The controller 41, which comprises a programmed CPU, exchanges data with the receiver 43 and the RF module 4 via a CPU bus line 5. The controller 41 also controls the RF module 4 to perform a frequency search process to switch between the received frequencies F1, F2 at a constant switching period T.sub.1. When the controller 41 detects the reception by the RF module 4 of a carrier sent from the road radio unit 100 in response to a level-detected signal 10 from the RF module 4, the controller 41 stops the frequency search process, and performs road-to-vehicle communications with the road radio unit 100 at a fixed frequency.
The receiver 43 detects a unique word from demodulated data 8 from the RF module 4 and checks the demodulated data 8 from the RF module 4 with a CRC (Cyclic Redundancy Check) code, and sends the detected result via the CPU bus line 5 to the controller 41.
The RF module 4 has a local oscillator with a frequency synthesizer. The RF module 4 receives the carrier sent from the road radio unit 100 at a frequency indicated from the controller 41 via the CPU bus line 5, and demodulates the received carrier. The RF module 4 has a level detector which outputs a level-detected signal 10 to the controller 41 when it detects the carrier sent from the road radio unit 100.
Operation of the conventional vehicle-mounted device 50 will be described below with reference to FIG. 2. When the controller 41 is turned on, the controller 41 performs the frequency search process to switch between the received frequencies F1, F2 at the constant switching period T.sub.1 (see FIG. 3 of the accompanying drawings). In the RF module 4, the frequency of the local oscillator is controlled so as to be able to receive the frequencies F1, F2. The switching period T.sub.1 is generated by an internal counter of the controller 41, and is set, in advance, to a suitable value according to the frame period of the carrier, which is of a time-division frame structure, sent from the road radio unit 100.
Operation of the conventional vehicle-mounted device 50 as the vehicle enters the communication range of the road radio unit 100 will be described below. It is assumed, for example, that the vehicle enters the communication range of the road radio unit 100 which is sending the carrier at the frequency F1.
When the vehicle with the vehicle-mounted device 50 enters the communication range of the road radio unit 100, the reception frequency of the vehicle-mounted device 50 is switching alternately between frequencies F1, F2 at the constant switching period T.sub.1 according to the frequency search process. When the reception frequency of the vehicle-mounted device 50 is tuned with the carrier frequency F1 sent from the road radio unit 100 as a result of the frequency search process and the level detector in the RF module 4 detects the carrier from the road radio unit 100, the RF module 4 sends a level-detected signal 10 to the controller 41. The controller 41 stops the frequency search process, and fixes the reception frequency of the RF module 4 to F1.
Subsequently, the carrier, which is of a time-division frame structure, sent from the road radio unit 100 is demodulated by the RF module 4 of the vehicle-mounted device 50. The receiver 43 detects a unique word from demodulated data 8 from the RF module 4 and checks the demodulated data 8 from the RF module 4 with a CRC code. The detected result is supplied to a reception management area in the receiver 43. The controller 41 monitors the reception management area via the CPU bus line 5. When the carrier is received well in as many successive frames as a back guard count (N1), the controller 41 judges the reception condition as the establishment of frame synchronization. After the establishment of frame synchronization, road-to-vehicle bi-directional data communications are carried out according to a time-division multiplex communication process.
When the vehicle-mounted device 50 finishes the bi-directional data communications and the vehicle runs out of the communication range of the road radio unit 100, the controller 41 attempts to detect a reception failure in the reception management area in the receiver 41. When the carrier is not received well in as many successive frames as a forward guard count (N2), the controller 41 judges the reception condition as the lack of frame synchronization. The controller 41 then restarts the frequency search process and enters a steady state. The above communication procedure is applicable in the communication range of the road radio unit 100 both at a toll gate and in a lane.
The vehicle-mounted device 50 of the conventional road-to-vehicle communication system repeats the frequency search process in order to detect the carrier sent from the road radio unit 100 at all times even outside of the communication range of the road radio unit 100. This is because the vehicle-mounted device 50 should start road-to-vehicle communications immediately when the vehicle-mounted device 50 enters the communication range of the road radio unit 100. Therefore, the controller 41 in the vehicle-mounted device 50 continues the frequency search process to switch between the received frequencies while operating the internal counter at all times after being initialized.
In the above ETC system, since the communication range of the road radio unit 100 is relatively narrow, the time spent for communications when the vehicle passes through a toll gate or the like is several 100 msecs. On the other hand, the time in which the vehicle-mounted device 50 moves outside of the communication range of the road radio unit 100 ranges from several minutes to several tens of minutes depending on different conditions.
While the controller 41 in the vehicle-mounted device 50 may perform road-to-vehicle communications only for several 100 msecs., the controller 41 thereby needs to operate at all times because of the frequency search process carried out at all times. With the controller 41 comprising a programmed CPU, the switching period T.sub.1 is counted by the internal counter thereof, and hence the controller 41 has a large power requirement.
Accordingly, the power consumption by the vehicle-mounted device 50 in its entirety remains essentially the same when the vehicle-mounted device 50 performs road-to-vehicle communications with the road radio unit 100 and when the vehicle-mounted device 50 carries out the frequency search process while the vehicle is outside of the communication range of the road radio unit 100. Thus, the vehicle-mounted device 50 causes a wasteful consumption of electric power. There has been a demand for lowering the power requirement of the vehicle-mounted device 50.